The present invention relates to an optical pickup apparatus capable of appropriately recording and or reproducing information onto or from optical information recording media by particularly using light sources having different wavelengths.
In recent years, the research and development of high density optical discs onto or from which information can be recorded or reproduced by using blue violet laser rays have been rapidly proceeded. For example, with regard to so called a Blue-ray Disc (it will be called BD from now on) onto or from which information is recorded or reproduced by using a light source having wavelength of 405 nm and an objective lens having NA of 0.85, it is possible to record 20-30 GB information onto a single side recording surface on an optical disc having diameter of 12 cm which is the same diameter of DVD (NA: 0.6, light source having wavelength 650 nm and a recording capacity: 4.7 GB). Also in the case of so called DH DVD (it will be called HD from now on) onto or from which information is recorded or reproduced by using a light source having wavelength of 405 nm and an objective lens having NA of 0.65, it is possible to record 15-20 GB information in a single side recording surface on an optical disc having diameter of 12 cm.
It is not good enough for the value of a product as an optical disc player or a recorder just having capability of appropriately recording and or reproducing information onto or from a single high-density optical disc. It is required to appropriately record and or reproduce information onto or from different kinds of high-density optical discs. Further, it is not good enough for the value of a product as an optical disc player or a recorder having only capability of recording and or reproducing information on to a high-density optical disc based on the fact that BD and HD will be sold in a mixed state in a marketplace. It is required to have capability of appropriately recording and or reproducing information onto or from the other high density optical disc in order to increase the product value of an optical disc player/recorder for high-density optical discs. It is difficult to form an appropriate focal spot onto each high density optical disc by using the same light converging optical system since the thickness of the protective layer of BD (t1) is 0.1 mm, the numerical aperture is 0.85, and the thickness of the protective layer of HD (t2) is 0.6 mm and the numerical aperture is 0.65. Further, there is a problem that the cost of the light converging optical system becomes high when separately providing light converging optical systems for BD and HD.
Japanese Patent Application Open to Public Inspection (TOKUHYOU) No. JP2004-516594 discloses an optical pickup apparatus capable of recording and or reproducing information onto or from DVD and DVR having different numerical apertures by using a diffraction grating for generating diffracted light rays having different order against polarization directions and a polarizing device with light rays having a single wavelength.
However, in the optical pickup apparatus disclosed in JP2004-516594, the diffraction grating is provided on the optical surface of an objective lens through which convergent light rays pass. The diffraction grating has a stepping structure having a zonal structure or a blaze type zonal structure. However, there is a problem that when passing the convergent light rays, the light ray amount of spot light rays becomes low resulting from shading or eclipse caused by the phenomenon that the light rays are blocked at the side of the zonal structure.